1. Field of the Invention
The present invention relates to an optical modulating element which is capable of changing, continuously or in stages, the wavelength of a transmitted light or a reflected light easily and at a high speed by controlling physical quantities such as voltage, temperature, etc. and electronic apparatuses such as direct viewing display apparatus, transmission display apparatus, image pickup apparatus, copying machine, printing & platemaking apparatus, facsimile communication apparatus, light communication apparatus or image processing apparatus, etc. constituted by using such optical modulating element.
2. Description of the Related Art
Liquid crystal display is utilized in a wide variety of fields such as watch, desk calculator, computer terminal, word processor, TV receiver, etc. As a typical display mode used for those purposes, there is the so-called TN (Twisted Nematic) mode in which the liquid crystal molecules in the liquid crystal cell are twisted by approximately 90.degree. as initial orientation. TN mode is designed to make a monochromic indication by disposing a liquid crystal cell between a pair of polarizers and by utilizing the optical characteristics of this cell i.e. the optically activating characteristics in the absence of application of display voltage and the racemizing characteristics in the presence of application of display voltage.
As for color display, the indication is made in multiple colors (indicated in prescribed plural number of colors) or in full colors (indicated in stepless colors) by providing a micro color filter of extremely small dimensions in red, blue and green for example and mixing colors by utilizing the light switching characteristics of TN mode. This principle is currently adopted for the display apparatus of a small liquid crystal TV receiver realized by application of active matrix drive or simple matrix drive.
As a display system widely used in the display apparatus for word processor, we may cite STN (Super Twisted Nematic) mode having a cell structure similar to that of TN mode and in which the twisting angle of the liquid crystal is set at 180.degree.-270.degree.. This mode is characterized in that the twisting angle of the liquid crystal is increased to no less than 90.degree. and the set angle in the direction of polarization of the polarizer is optimized to reflect the sudden deformation in the molecular orientation due to increase of applied voltage in the change of birefringence of the liquid crystal so as to realize electro-optical characteristics having a sharp threshold value. It is therefore suitable to simple matrix drive.
On the other hand, this mode has the drawback of presenting a yellow-green or dark blue tint as the background color for indication because of birefringence of the liquid crystal. As a method to improve this, there is a technique which consists in correcting the color by placing an optical compensation panel or a phase difference plate formed by high polymer molecules such as polycarbonate, etc. on the STN panel for indication to enable monochromic indication, and a liquid crystal display apparatus (LCD) of such structure is currently on the market under the name of "Paper White LCD". For color indication of this apparatus, an indication in multiple colors and in full colors is possible based on an operating principle similar to that of the TN mode.
For services requiring a wide visual angle, the so-called GH (Guest Host) mode is used in which a pigment with different absorbencies in the direction of major axis and the direction of minor axis (dichromatic pigment) is added to the liquid crystal. This system can be classified into Heilmeyer type which uses a polarizer, White/Tiller type (phase transfer type) which does not use any polarizer and 2-layer type, etc. In any case, the operating principle of the system consists in controlling the orientation of pigments through the orientation of liquid crystal molecules by voltage to use the difference of absorbency in the direction of pigment molecules for the indication. Color indication becomes possible by either using pigments which absorb a part of the visible light or combining a colored filter with a GH cell using a pigment which turns into black.
Another method of color indication consists in installing an element capable of controlling the wavelength of the transmitting light on the front face or the rear face of a monochromic display and switching the wavelength of that transmitting light sequentially to obtain a color indication. As an example of application of this method to a projection display, there is a technique of sealing a high polymer molecule distributed liquid crystal in a CdSe-TFT panel (TFT=thin film transistor), installing a disc provided with red, green and blue filters in front of a light source and making an indication on the TFT-LCD in synchronization with the change in color of the illuminated light by the rotation of the disc to obtain a full-color indication. On the other hand, a color indication is also progressing in electronic apparatus other than display apparatus such as image pickup apparatus (camera), color sensor, copying machine, etc.
Image pickup apparatus is currently forming a large market as the performances of CCD (=charge coupled device) improve with the progress of semi-conductor manufacturing techniques. Especially, loading of CCD in video cameras is greatly contributing to the expansion of the home use market.
The demand is increasing rapidly also for color sensors as they came to be loaded on video camera for adjustment of white balance although they were used only for industrial purposes in the past.
Those electronic apparatuses are provided with a color filter consisting of a synthetic resin material, etc. on the front face of the photoelectric transfer face and constructed in a way to calculate the characteristics of the entire light of incidence by detecting the characteristics of specific wavelength components of the light incident on the photoelectric transfer element.
What is important about the structure of an electronic apparatus having such photoelectric transfer element is the structure of the color filter in which filters corresponding to the wavelength components of light or red (R), green (G) and blue (B) are placed in parallel on the photoelectric transfer face in the same way as in the display apparatus. Namely, the three filters of red, green and blue constitute one picture element on the display.
In addition to the method of thus decomposing the light of incidence into 3 primary color components, there is also a proposal of a construction in which filters of auxiliary colors only (cyan, magenta and yellow) or 3 primary colors mixed with those auxiliary colors are placed in parallel.
In a copying machine, a full color indication is reproduced by irradiating a white color on the document to be copied, writing optical images of 3 primary colors on a photosensitive drum by switching the red, green and blue filters of reflected light sequentially by using a mechanical construction, and then transferring those optical images on the recording paper a plural number of times through the intermediary of a color developer. In this principle, full color copying is also possible by irradiating in advance the source lights of 3 primary colors on the document to be reproduced by using color filters of red, green and blue, decomposing the copy image into 3 primary colors and then treating them on the photosensitive drum in the same way as above. The sequential switching control of color filters in this case is performed mechanically in normal cases.
In a digital copying machine, the image of the copy to be reproduced is submitted to photoelectric transfer by means of a solid state imaging element (CCD) and then to treatment of digital image signal to reproduce the original picture in color. The solid state imaging element used in this case is the same as the CCD and its operating principle also follows the explanation given earlier.
As mentioned above, color indication constitutes one of the development trends of today in various display apparatuses or image input/output apparatuses such as imaging element, color sensor, copying machine, etc. However, the current technology of color indication still leaves a lot of problems to be solved.
Namely, regarding display apparatuses, we know a structure in which micro color filters of red, green and blue, for example, are provided for each picture element to be displayed on the front face of a liquid crystal display apparatus making a monochromic indication for the purpose of realizing a multicolor or full-color indication. In such a panel structure, a drop of resolution is produced because micro color filters of a plural number of lights are combined into one picture element to be displayed. In the case where you reduce the size of picture element to be displayed of the liquid crystal display apparatus making the indication to prevent that problem, an increase of resistance is produced because of the reduction in size of the electrode through which the indication signal flows and this leads to the production of unevenness in the density of image displayed between the position where the signal is fed and a position apart from the position of signal feed in the liquid crystal display apparatus. Moreover, it is also necessary to set the manufacturing environments at an extremely high degree of purity and this produces problems such as drop of yield ratio in manufacture under the influence of dust, etc.
On the other hand, in a display apparatus using the GH (guest host) mode, we know a technology of using pigments which absorb a part of the visible light to realize a multi-color or full-color indication. In such a case, it is necessary to place guest host liquid crystals supplemented with different kinds of pigment, but this technology has a problem that the picture elements to be displayed of each layer do not agree with one another because of parallax error when they are seen from a diagonal direction. A solution to this problem may be given by a technology of reducing the thickness of the glass substrate of each block stacked panel. In this case, however, the glass substrate is extremely liable to be broken in the manufacturing process of the liquid crystal display apparatus, presenting a problem of difficulty of handling. Moreover, in a guest host liquid crystal using pigments capable of displaying the black color, it is necessary to use micro color filters as mentioned before and this leads to the basic problem of drop in resolution.
On the other hand, in the technology of applying the red, green and blue lights to display apparatus by switching them sequentially, the resolution improves compared with the case of use of micro color filters because one picture element of the display apparatus directly turns into the picture element to be displayed. In the past, however, there was a problem with the miniaturization or the durability of the apparatus because the switching of colors is realized with a mechanical construction by providing color filters of 3 colors in one same direction on a disc for example and driving the disc for turning, etc. It also has a problem of noise.
On the other hands, in image pickup apparatus or the color sensors, there is a problem of drop in resolution as in display apparatuses because of the structure of providing micro color filters. Another problem is that the structure of the apparatus gets complicated because of use of micro color filters of fine structure.
In the color copying machine, the switching operation of filters for decomposing the light irradiated on the document or the reflected light from the document into 3 colors is performed either mechanically or by providing separate light sources for the respective colors as mentioned before, and this leads to problems such as increase in size, drop in resolution or noise as mentioned before. When providing separate light sources for the respective colors, there are such problems as increase in the size of structure, service life of light source, purity of color and (increase of) power consumption.